Questions about filtering bridges

Richard Coleman rcoleman at
Sun Sep 16 19:58:00 PDT 2007

I'm setting up a filtering bridge and have a couple questions. 
Hopefully someone here can help.  I've looked at all the docs online 
(and lots of Google searches) but there isn't much recent info on 
filtering bridges.

The setup is pretty simple: fxp0 is external and fxp1 is internal.

# rc.conf
ifconfig_bridge0="addm fxp0 addm fxp1 up"

Question 1: In the Handbook section on bridging, it says that if you 
need to setup an ip address, you should put it on the bridge interface 
(bridge0).  But in the OpenBSD docs on filtering bridges, they say to 
put it on the inside interface.  What are the consequences of doing it 
either way?

Questions 2: If I use the following pf.conf (should block everything 
inbound, but allow everything outbound), I notice I'm still able to ssh 
into the bridging firewall itself.  Why isn't that blocked?  I'm 
guessing it's a consequence of the fact that I put an ip address on the 
bridging interface, but I'm not sure.  What am I missing?

# pf.conf

# interfaces

# options
set skip on lo0
set block-policy drop

# normalization
scrub in on $ext_if all
scrub out on $ext_if random-id

# external interface, inbound
# default is to block all inbound on external interface
block in log on $ext_if all

# external interface, outbound
block out log on $ext_if all
pass out on $ext_if proto tcp all flags S/SA keep state
pass out on $ext_if proto { udp, icmp } all keep state

# internal interface, inbound
pass in on $int_if all

# internal interface, outbound
pass out on $int_if all

Richard Coleman
rcoleman at

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